1. Technical Field
The present invention relates to noise prevention techniques in a lighting apparatus which uses a fly-back type DC voltage converting means.
2. Related Art
When a discharge lamp (HID lamp or the like) or a light emitting element (LED or the like) is used for car illumination, a related art switching power supply circuit (DC-DC converter) is employed for controlling the power of the light source. For example, in a related art fly back scheme which employs a transformer, a semiconductor switching element such as an FET arranged on the primary side of the transformer, there are three operation modes as shown below:                Current Continuous Mode;        Current Boundary Mode; and        Current Discontinuous Mode.        
In the current continuous mode, a switching element transits to the on position before energy accumulated in a transformer is completely released to the secondary side. On the other hand, in the current boundary mode, a switching element turns on at the time energy accumulated in a transformer has been completely released to the secondary side. In the current discontinuous mode, a switching element turns on after the lapse of a certain period (discontinuous period) from the time energy accumulated in a transformer is completely delivered to the secondary side.
For operating a switching element at high frequencies, the current boundary mode is advantageous in terms of circuit efficiency, because of a reduction in reverse recovery loss of a rectifier diode on the secondary side. However, when constant power is applied to a discharge lamp, the switching frequency is uniquely determined, and the harmonics of the switching frequency exert a problematic influence (radio noise) on a radio broadcasting band.
Thus, a related art method is employed for intentionally generating fluctuations in frequency to disperse the frequency spectrum so as to reduce a temporal average value of the noise.
In a configuration which generates fluctuations to change a switching frequency in the control of an output current or power associated with a discharge lamp (see, for example, Japanese patent publication JP-A-2003-264095), for example, a related atr method changes a reference voltage of an error amplifier, which forms part of a control circuit, by a predetermined frequency, or changes an output voltage of the error amplifier by a predetermined frequency.
However, in the related art circuit configurations, problems arise in terms of the difficulties of accurately setting a fluctuation width for target power in design. Further, in a more complicated configuration, there is an increase in cost and the like associated with the setting.
The radio broadcasting band is divided into the LW band, AM band, SW band, FM band, and the like. For example, to rely on a high frequency switching technique to accomplish a reduction in a parameter such as the size of a circuit device in car illumination applications, the frequency must be set in a band located between the LW band and AM band (approximately 300 to 500 KHz). In this event, unless the fluctuation width is not appropriately set, the noise level cannot be sufficiently reduced, thus requiring countermeasures through additional provision of a noise filter, leading to an increase in the number of parts and the cost.